There has been known a decolorizing device for decolorizing an image on a sheet by subjecting the sheet including the image formed thereon with a decolorable colorant to a decolorizing process such as heating so as to turn the decolorable colorant in a colored state into a decolorized state. Some of such decolorizing devices read an image on a sheet and save the image data before performing the decolorizing process. Moreover, in some of such decolorizing devices, a side having been subjected to the decolorizing process is read after the decolorizing process of the image, and a destination of sheet discharge is sorted into either a reusable sheet stacking section for stacking reusable sheets or a rejected sheet stacking section for stacking non-reusable sheets based on determination if there is an undecolorized portion or not.
In order to perform both of the image data saving before the decolorizing process and the determination regarding the existence of an undecolorized portion, image reading needs to be performed before and after the decolorizing process. In such a case, however, there is a problem of an increase in time to finish the series of decolorizing processes.